1. Field of the Invention
The present invention relates to a method of producing butanol by separating butanol from a butanol-containing solution. More particularly, the present invention relates to a method for producing butanol, which method comprises a step of removing inorganic salts, sugars, proteins, catalytic components and/or the like remaining in a butanol-containing solution with a nanofiltration membrane.
2. Description of the Related Art
Butanol is a compound which is industrially very important as a raw material of chemicals and pharmaceutical agents, as a solvent, and as a fuel material. It is well known that butanol can be industrially synthesized from acetaldehyde by the Wacker process, or from propylene, carbon monoxide and water by the Reppe process, and it also has long been known that these can be produced by acetone-butanol fermentation. Production of butanol by acetone-butanol fermentation have problems in that the cost of the substrate to be used as the nutrient source for microorganisms and the cost of purification of the product are high, so that production of butanol has become dependent on chemical synthesis. However, because of the recent decrease in the crude oil resource and substantial rise in its price, production methods of butanol using biomass have been expected to be useful as alternatives to chemical synthesis, and therefore reduction of the production cost in acetone-butanol fermentation is demanded.
In general, as the method for purifying butanol, solvent extraction or distillation is employed. In solvent extraction, in cases where the desired product is a lower alcohol, which is highly soluble in water, distribution of the lower alcohol into the organic phase is difficult, so that use of a special extraction solvent or multistep extraction may be required, which leads to increase in the cost (Patent Document 1). Further, in purification by distillation, since the concentration of butanol in the fermentation broth is low, water, whose boiling point is lower than that of butanol, needs to be distilled off in a large amount, so that a method that enables efficient concentration of butanol is demanded. As a solution to solve the problem, a method for concentrating a fermentation alcohol using a separation membrane has been devised. In Patent Document 2, a method for concentrating an alcohol by the pervaporation separation method using a silicone rubber membrane is disclosed. However, this method is the so called batch filtration concentration wherein purified fermentation broth is recovered from a fermenter and then treated in a pervaporation apparatus under specific conditions, which is not rational in view of device configuration.
Examples of the continuous method for alcohol concentration as an alternative to the batch process include the method using a silicalite membrane coated with silicone rubber, which is disclosed in Patent Document 3. In cases where the operation of concentration is carried out by introducing a silicalite membrane into a fermenter, fouling occurs with time due to organic acids such as succinic acid and malic acid produced as by-products. By coating the surface of the silicalite membrane with the silicone rubber to make the membrane surface hydrophobic, this fouling is suppressed. However, since fouling substances accumulate with time also on the surface of the silicone rubber, the performance of the separation membrane may decrease with time.
Further, in case where an alcohol is distilled from a fermentation broth, sugars, amino acids, side metabolites such as organic acids, and the like remaining in the fermentation broth are heated to yield by-products, which are then contaminated in the distillate fraction as impurities, which is problematic. Therefore, purification of the fermentation broth is also an important problem. As methods for purifying an alcohol, a method for purifying 1,3-propanediol wherein distillation is carried out in combination with microfiltration, ultrafiltration, nanofiltration or ion exchange (Patent Document 4); and a method for separating diol using a reverse osmosis membrane or a nanofiltration membrane (Patent Document 5) are disclosed. However, these prior arts do not disclose the effect of difference in the material of the nanofiltration membrane on the permeation selectivity and on purification of butanol.